OBJECTIVE: LEARN ABOUT NEWTON’S FIRST LAW AND UNDERSTAND THE VOCABULARY AGENDA: START POWERPOINT NOTES

CHAPTER 2: NEWTON’S FIRST LAW OF MOTION- THE LAW OF INERTIA

ARISTOTLE’S CLASSIFICATION OF MOTION Motion requires a force He classified motion in natural motion and unnatural motion Natural: (he believed) occurred without force EX: Motions of Sun, Moon, and other objects in the sky Did them on their own Unnatural: requires force such as push or pull by animals or people

GALILEO’S CONCEPT OF INERTIA Aristotle's idea was taken for 2000 years, but Galileo proved this false He found: Force is required to start an object moving, but one moving, no force is required to keep It moving Except for force needed to overcome friction (Chp. 3) Inertia: If moving, tend to remain moving. If at rest, tend to stay at rest. Property of matter to resist changes in motion

GALILEO’S CONCEPTS OF SPEED AND VELOCITY

VELOCITY Velocity: know speed and direction of an object Speed is a description of how fast, velocity is how fast and in what direction Vector Quantity: specifies direction and magnitude Constant Speed: steady speed Constant Velocity: constant speed and constant direction Constant Direction: Object moves in a straight line and doesn’t change direction

CONCEPT CHECK What is the average speed of a cheetah that sprints 100m in 4 seconds? What about if it sprints 50m in 2s? The speedometer on a bicycle moving east reads 50 km/h. It passes another bike moving west at 50 km/h. Do both bikes have the same speed? Do they have the same velocity? “The bird flies at constant speed in a constant direction.” Say the same sentence in fewer words.

MOTION IS RELATIVE Always moving! Relative motion: motion relative to something else EX: car is moving 25 mph relative to the road Galileo was concerned with how things move rather than why they move

NEWTON'S FIRST LAW OF MOTION Isaac Newton, at age 24, extended Galileo’s concept of inertia Newton’s laws were influenced by Galileo’s findings Newton’s First Law (law of inertia): “Every object continues in its state of rest or uniform speed in a straight line unless a net force acts on it” Rest stays at rest! Motion stays in motion!

CONCEPT CHECK When the space shuttle moves in a nearly circular orbit around Earth, is a force needed to maintain its high speed? If suddenly the force of gravity were cut off, what type of path would the shuttle follow?

NET FORCE: THE COMBINATION OF ALL FORCES THAT ACT ON AN OBJECT Often more than one force acts on an object EX: Throwing a basketball, the force of gravity, air friction, and the pushing force you apply with your muscles all act on the ball Net Force: ball is the combination of all these factors Net force changes an object’s state of motion EX: pull a 5 lb. weight, if a friend also pulls with 5 lbs., than the total net force would be 10 lbs. Force is in the unit of N (newton) You show the amount of force by the length of arrows

EQUILIBRIUM FOR OBJECTS AT REST Newton is a unit of weight Hang a sugar bag upside down: 2 forces acting on it: 1.Tension force: acting upward on the bag 2.Weight: acting downward Equal but opposite forces = 0 Mechanical Equilibrium: state where no physical changes occur; it is a state of steadiness Net force = 0 Equilibrium Rule: When an object is said to be in mechanical equilibrium

EQUILIBRIUM RULE The vector sum of Forces

CONCEPT CHECK A gymnast is hanging from the rings: 1.If she hangs with her weight evenly divided between the two rings, how would scale readings in both supporting ropes compare with her weight? 2.Suppose she hangs with slightly more of her weight supported by left ring. How would a scale on the right read?

SUPPORT FORCE: WHY WE DON’T FALL THROUGH THE FLOOR Support force: the upward force that balances the weight of an object on a surface An object at rest on a horizontal surface, the support force must equal the object’s weight Ex: Spring on hand Ex: Measuring your weight on a scale

CONCEPT CHECK 1.What is the net force on a bathroom scale when a 110 pound person stands on it? 2.Suppose you stand on two bathroom scales with your weight evenly distributed between the two scales. What will each scale read? How about if you learn to put more of your weight on one scale than the other?

EQUILIBRIUM FOR MOVING OBJECTS An object moving at constant speed in a straight-line path is also in equilibrium Sum still equals 0 (Newton’s First Law) Must not be changed by any force or else the object is not in equilibrium Objects at rest are said to be in static equilibrium; objects moving at constant velocity are said to be in dynamic equilibrium. Types of mechanical equilibrium

CONCEPT CHECK An airplane flies at constant velocity. In other words, it is in dynamic equilibrium. Two horizontal forces act on the plane. One is the thrust of the propeller that pushes it forward. The other is in the force of air resistance (drag) that acts in the opposite direction. Which force is greater?

EARTH MOVES AROUND THE SUN The law of inertia states that objects in motion remain in motion whenever no net force acts. EX: Stand up